Power supply for telecommunications device

ABSTRACT

A power supply has a plurality of single line interface circuits ( 20, 22, 24  and  26 ) each connected by a respective line, to a respective telephone ( 4, 6, 8  and  10 ) or other telecommunications device. Each SLIC applies a sufficient voltage derived from a battery to the line to operate the respective telephone, and dissipate any excess power supplied by the battery. The apparatus includes common power regulation means ( 30 ), which incorporates a switch mode power supply unit ( 68 ), and which monitors the voltages being supplied by the SLICs at any one time and reduces the voltages applied to the SLICs to substantially the maximum voltage required by any one of the SLICs, thereby reducing the amount of power dissipated by the SLICs.

FIELD OF THE INVENTION

This invention relates to power supply apparatus for supplyingelectrical power to a number of remotely situated telecommunicationsdevices, such as telephones, and to an assembly comprising suchapparatus connected to a plurality of telephones.

BACKGROUND OF THE INVENTION

A conventional telephone exchange can serve a large number of telephonesconnected to the exchange along corresponding lines which are ofdiffering lengths as a result of differences in the distances of thetelephones from the exchange. As well as relaying telecommunicationssignals, the exchange also feeds to each line sufficient power tooperate the telephones. That power can vary from one telephone toanother in view of the differing lengths of the lines, which give riseto differences in the amount of resistive losses on the various lines.

The variation in power requirements is accommodated by means of a numberof subscriber line interface circuits (SLICS), each of which relaysthrough a respective line a sufficient amount of power to operate itsrespective telephone. If linear SLICs are used, excess power supplied toeach circuit from the source is dissipated as heat. In this case, thevoltage applied to each SLIC from the power source has to be sufficientto operate the telephone connected by the longest line. If a largenumber of other telephones are connected on much shorter lines, arelatively large amount of power has to be dissipated by the SLICsassociated with the shorter lines, which can be uneconomical and leadsto cooling problems in the exchange.

Some exchanges use SLICs which have individual internal switch modepower supply units which reduce the power dissipation. However, suchSLICs are relatively expensive.

SUMMARY OF THE INVENTION

According to the first aspect of the invention, there is provided powersupply apparatus for connection to a plurality of remotely situatedtelecommunications devices along corresponding lines, the apparatuscomprising a plurality of interface circuits, each having an input forconnection to a common power source and an output for connection to arespective line, each interface circuit, when active, feeding sufficientpower, derived from the source, to the line to provide the requiredamount of power to operate the respective telecommunications device,dissipating any excess power supplied to the interface circuit from thesource, wherein the apparatus includes common power regulation means formonitoring a characteristic of the amount of power being fed by eachactive interface circuit to its respective line and so controlling thepower supplied to each active interface circuit that said supplied poweris substantially equal to the largest of the amounts of power requiredby the active interface circuits.

Thus the power regulation means ensures that the power supplied to allthe interface circuits is governed by the requirements of the activeinterface circuit which has to feed the most power to its respectiveline. The invention therefore helps to reduce the amount of power whichneeds to be dissipated by the active interface circuits, particularlywhen the interface circuits with the highest power requirements are notactive, and thus enables the apparatus to use relatively cheap linearSLICs, whilst avoiding, or at least ameliorating problems arising fromthe power dissipation in the SLICs.

Conveniently, the characteristic monitored by the regulating means isthe respective voltage applied by each active interface circuit to itsrespective line, and the regulating means controls the power supplied tothe interface circuits by controlling the voltage applied to the inputsof the interface circuits.

Conveniently, the apparatus is adapted to be connected to a source of dcpower, for example, a battery.

The power regulation means preferably includes a switch mode powersupply unit operable to reduce the voltage supplied by the dc source tothe interface circuits.

Preferably, the interface circuits are mounted on a single card formounting in a rack.

Preferably, the apparatus has a plurality of such cards, and theregulating means is one of a plurality of such means each associatedwith a respective card.

The use of a plurality of cards and associated regulating means enables-he allocation of lines to interface circuits to be such that theinterface circuits on each card are connected to a number of lines ofsimilar lengths, thereby reducing the range of powers to be fed by theinterface circuits in each respective card. That reduction in rangeenables the amount of power dissipated by the interface circuits to befurther reduced.

According to the second aspect of the invention, there is provided atelephone exchange having power supply apparatus in accordance with thefirst aspect of the invention connected to a plurality of remotelysituated telephones along corresponding lines which are not necessarilyof the same resistance, and to a dc power source.

Preferably, the apparatus has a plurality of cards, each having arespective set of interface circuits and all the lines connected to thecircuits on each cards are of lengths which lie in a respective one of aplurality of different ranges.

The invention also lies in power regulation means for use in apparatusin accordance with the first aspect of the invention, the powerregulation means comprising a power input for connection to a battery, aplurality of further inputs, each for connection to the output of arespective interface circuit, monitoring means connected to said furtherinputs for monitoring the voltage, or current fed, by each activeinterface unit to a respective line connecting that interface unit witha respective telecommunications device, the monitoring means beingoperable to determine, at any one time, the maximum of said voltages orcurrents; and control means for applying to all said interface circuitsan output voltage, derived from said battery voltage, substantiallyequal to the largest of the voltages required by an active interfacecircuit.

Preferably, the monitoring means is arranged to monitor the voltagesapplied by said active interface circuits and to determine the largestof said voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

Power supply apparatus in accordance with the invention will now bedescribed, by way of example, with reference to the accompanyingdrawings in which:

FIG. 1 is a schematic block diagram of the apparatus when connected to anumber of telephones;

FIG. 2 is a block diagram of an interface circuit forming part of theapparatus shown in FIG. 1;

FIG. 3 is a block diagram of another part of the apparatus;

FIGS. 4 and 5 are diagrams illustrating the operation of the interfacecircuits;

FIG. 6 shows an alternative type of interface circuit for use inapparatus not in accordance with the invention; and

FIG. 7 shows an exchange having a subrack containing a plurality ofpower supply apparatuses in accordance with the invention.

DETAILED DESCRIPTION

The power supply apparatus shown in FIG. 1 forms part of a telephoneexchange 2 which is connected to a number of telephones, 4, 6, 8 and 10along corresponding lines 12, 14, 16 and 18, each comprising a pair ofconductors (A and B). For the sake of clarity, only 4 telephones areshown in the drawing, although typically the apparatus will servebetween 6 and 15 telephones (or other telecommunications devices such asfax machines).

The apparatus in this example operates under a constant currentprotocol, which applies in the UK, by supplying a predetermined currentto each telephone which is in use. However, apparatus in accordance withthe invention can operate under different protocols, for example, theconstant resistance requirements applicable in the USA.

The telephones are located at various different distances (in the range0-5 km) from the exchange 2, and as a result, the connecting lines arenot all of the same length. However, the lines have the sameresistivity, and therefore do not house the same resistance.

Each of the lines 12, 14, 16 and 18 is connected to a respective one offour identical linear subscriber line interface circuits (SLICs)referenced 20, 22, 24 and 26. The subscriber line interface circuits allhave inputs for the electrical power needed to operate the telephones 4,6, 8 and 10, and those inputs are connected in parallel, as indicated byline 28 to regulating means 30 in the form of a compensating powersupply.

The single line interface circuit 20 is shown in greater detail in FIG.2, in which the resistor Rloop represents the resistance of the line 12and telephone 4. The circuit 20 comprises an amplifier 50 controlled bya feedback loop which includes an anti-saturation circuit 52 and anexternal programming resistor Rdc. The amplifier 50 applies a voltage,Vab, to the line 12 causing a current to flow therealong.

In the case of a constant current system, the current supplied by theamplifier is fixed unless the anti-saturation circuit 52 is activated.In the case of a constant resistance system, the voltage at the outputof the amplifier 50 is fed back, as indicated by the dotted line 54, tosimulate a constant resistance. Again, this may be modified by theanti-saturation circuit 52 (in the case of longer lines).

The anti-saturation circuit 52 provides a constant headroom voltage forthe transmission of voice frequency (VF) signals, and will, ifnecessary, reduce the current (and hence Vab) to maintain voltageheadroom for VF signals. The amplifier 50 is supplied with a voltageVreg by the regulating means 30, and the difference between Vreg and Vabis dissipated in the amplifier 50.

With reference to FIG. 3, the compensating power supply 30 has an input32 for connection to a battery (not shown) or other source of dc power,and four monitor inputs 56, 58, 60 and 62, each of which is connected tothe output of a respective one of the interface circuits 20, 22, 24 and26. Each of the monitor inputs is in turn, connected to monitorcircuitry 64 which monitors the voltages, V(AB) 1-4, being applied toeach interface circuit to its respective line (when the telephoneconnected thereto is in use), determines the maximum of those voltagesand feeds a signal representative of that maximum to a voltage controlunit 66. The unit 66 is in turn connected to a power supply switchingregulator 68 which is also connected to the power supply input 32, andto an output 70 (connected to the line 28). The regulator applies to theline 28 a voltage which is less than or equal to the battery voltage,and which is controlled by the unit 66 so as to be related to themaximum voltage in a way described below.

In this example, the constant current requirement is used to determinethe voltage which has to be supplied by each SLIC to its respectiveline, when in use; the voltage applied has to be sufficient to produce30 mA of current along the respective line. In view of the differencesin resistances of the lines 12, 14, 16 and 18, that voltage will varyfrom one SLIC to another. Since each SLIC is a linear device, any excesspower (i.e. power which is not then transmitted along the respective oneof the lines 12, 14, 16 and 18) is dissipated as heat.

In use, switching regulator 68 of the compensating power supply 30controls the voltage applied to the line 28 so that it is equivalent tothat maximum voltage plus a bias voltage needed in order for the SLIC tooperate. As a result, the power dissipated by the active SLIC which isapplying the largest voltage to its respective line is minimised, andthe power dissipated in the other active SLICs is less than would be thecase if the full battery voltage were applied to all the SLICs.

It will be appreciated that the voltage supplied along the line 28 canvary depending upon which of the SLICs is in use at any one time.

As shown in FIG. 7, the components of the power supply apparatus can bemounted on a single card 74 accommodated in a rack 76 of a plurality ofsuch cards, 74, 78, 80, each of which serves a respective set oftelephones. The allocation of telephones to cards is such that thedistance of each member in a set from the exchange lies in a respectiverange associated with that set, so that the range of different voltageswhich have to be applied by the SLICs on any one card can, as far as ispracticable, be kept to a minimum. For example, all the telephonesconnected to the card 74 are situated more than 1 km from the rack 76,the telephones connected to the card 78 are situated in the range 300 mto 1 km and the telephones connected to the card 80 in the range 0 to300 m.

A number of examples of the operation of the power supply apparatus onany one card, and of the resultant savings in dissipated power, arediscussed below.

As explained above, all of the monitor inputs V(AB) 1-4 are examined andthe output of the supply 30 is then adjusted to supply the voltage whichwill maintain the required subscriber line conditions on the worstactive line (i.e. the line with the highest resistance). Some examplecases are given below in which the battery voltage is 48 volts, V(ab) isthe voltage applied to the line 28, (and is equal to the voltagedetected at the respective one of the inputs 50, 58, 60, 62) and whichassume a requirement of 30 mA constant current, telephone resistance of400 ohms, subscriber line resistance of 200 ohms/km, and 5V biasrequired for line drivers.

With reference to FIG. 4, Rloop is the sum of the resistances of thetelephone and the line, Rline and Rphone. Vab is the voltage which theSLIC has to apply to its line and Vreg is the voltage supplied to theSLIC by the supply 30.

Case (a)

All subscribers active on very short lines (negligible line resistance).

Vab (max)=0.03×Rloop=0.03×400=12V.

Vreg=(0.03×400)+5=17V.

In this case V(reg) will be set to 17V, and the dissipation in each SLICwill be 0.15W (total of 0.6W for all 4 channels)

Without compensating power supply, the power dissipated would be0.03×(48−(0.03×400)=1.08W for each channel, would have to be dissipated

Power saving is equivalent to 0.93W per channel.

Case (b)

3 subscribers active on very short lines (12, 14, 16-negligible lineresistance), one subscriber active on 300M line (18).

V(ab){12, 14 and 16}=(0.03×400)=12V.

V(ab){18}=(0.03×(400+(0.3×200))=13.8V.

Vreg=13.8+5=18.8V.

In this case v(reg) will be set to 18.8V, and the dissipation in SLICs20, 22, 24 will be 0.204W and in SLIC 26 will be 0.15W (total of 0.762Wfor all 4 channels).

Without compensating power supply, the power dissipated would be0.03×(48−(0.03×400))=1.08W for SLICS 20, 22 and 24 and0.03×(48−(0.03×(400+(0.3×200))))=1.026W for SLIC 26.

Power saving is equivalent to 0.876W per channel.

Case (c)

3 subscribers active on very short lines (12, 14, 16) (negligible lineresistance), one subscriber active on 3 km line, (18)

V(ab){12, 14 and 16}=(0.03×400)=12V

V(ab){18}=(0.03×(400+(0.3×200))=30V

V(reg)=30+5=35V.

In this case V(reg) will be set to 35V, and the dissipation in SLIC 20,22 and 24 will be 0.69W and in SLIC 26 0.15W (total of 2.22W for all 4channels).

Without compensating power supply the power dissipated would be0.03×(48−(0.03×400))=1.08W for SLICs 20, 22 and 240.03×(48−(0.03×(400+(0.3×200))))=0.54W for SLIC 26.

Power saving is equivalent to 0.39W per channel.

Case (d)

1 subscriber active on very short line 12 (negligible line resistance),three subscribers active on 3km lines (14, 16, 18).

V(ab){12}=(0.03×400)=12V.

V(ab){14, 16, 18}=(0.03×(400+(0.3×200))=30V

V(reg)=30+5=35V.

In this case V(reg) will be set to 35V, and the dissipation in SLIC 20will be 0.69W and SLIC 22, 24, 26 is 0.15W (total of 1.14W for all 4channels).

Without compensating power supply the power dissipated would be0.03×(48−(0.03×400))=1.08W for SLIC 20 and0.03×(48−(0.03×(400+(0.3×200))))=0.54W for SLICS {22, 24, 26}.

Power saving is equivalent to 0.39W per channel.

When the apparatus is used in a constant resistance system, the modelillustrated in FIG. 5 applies. The following calculations assume arequirement of 2×200 ohm constant resistance feed (Rfeed) from 50V(Vfeed), a telephone resistance (Rphone) of 400 ohms, a line resistance(Rline) of 200 ohms/Km (multiplied by the line length), and a 5V biasrequirement for the line drivers. Again, Vab is the voltage applied by aSLIC to its line and Vreg is the voltage applied to the SLICS by thesupply 30. lloop is the current flowing along the line.

Case (e)

All subscribers active on very short lines (negligible line resistance).

 Vab=50×400/(400+400)=25V

lloop=Vab/Rloop=25/400=62.5mA

Vreg=25+5=30V

In this case Vreg will be set to 30V, and the dissipation in each SLICwill be 0.3125W (total of 1.25W for all 4 channels).

Without compensating power supply the power dissipated would be0.0625×(48−25)=1.44W for each channel.

Power saving is equivalent to 1.125W per channel.

Case (f)

3 subscribers active on very short lines (negligible line resistance),one subscriber active on 3 km line.

Vab{1}=50×(400+600)/(400+600+400)=35.7V

lloop=Vab/Rloop=35.7/(400+600)=35.7mA

Vab{2, 3, 4}=50×400/(400+400)=25V

lloop=Vab/Rloop=25/400=62.5mA

Vreg=35.7+5=40.7V

In this case Vreg will be set to 40.7V, and the dissipation in eachSLIC{1} will be 0.179W and in SLICs {2, 3, 4} will be 0.981W per channel(total power=3.122W).

Without compensating power supply the power dissipated would be0.0357×(48−35.7)=0.439W for channel 1 and channels 2, 3, 4power=0.0625×(48−25)=1.44W per channel (total power=4.76W).

Power saving is equivalent to 0.41W per channel.

An alternative control loop method would employ the SLIC output signal(usually available on commercial components) which gives an indicationof the loop current. In constant resistance feed applications theminimum loop current will directly show the longest line and hencehighest Vab.

For constant current feed, where all lines are in the constant currentregion, the loop currents will be equal, in this case the controlvoltage would be reduced until one of the SLIC's anti-saturationcircuits (the one with the longest line) began to reduce the loopcurrent. Consequently the corresponding monitor input would begin tofall below the constant current value and at this point the regulatedsupply would maintain its voltage keeping the SLIC just on the edge ofthe constant current and anti-saturation regions of operation. Themonitor block in this case outputs the minimum voltage (Vmin).

A control voltage (Vreg) is produced from the maximum/minimum loopmonitor voltage to enable adjustment of the switching regulator tosupply the voltage which will maintain the required subscriber lineconditions on this worst case line. This block also ensures theregulated voltage is within SLIC battery input operating range.

The SLIC shown in FIG. 6 is of a type having its own built-in regulatorwhich operates, in a similar fashion to regulator 68, to reduce thevoltage applied from a battery, without dissipating excess power. Anexchange incorporating such SLICs, however, is relatively costly.

Although the invention has been described in relation to telephoneexchanges, it can also be used in various types of primary multiplexingequipment, such as primary multiplexers supplying subscriber line accessand situated remotely from the telephone exchange.

What is claimed is:
 1. Power supply apparatus for connection to aplurality of remotely situated telecommunications devices alongcorresponding lines, the apparatus comprising a plurality of interfacecircuits, each having an input for connection to a common power sourceand an output for connection to a respective line, each interfacecircuit, when active, feeding sufficient power, derived from the source,to the line to provide the required amount of power to operate therespective telecommunications device, and dissipating any excess powersupplied from the source, wherein the apparatus includes common powerregulation means for monitoring a characteristic of the amount of Dowerbeing fed by each active interface circuit to its respective line and socontrolling the power supplied to each active interface circuit thatsaid supplied power is substantially equal to the largest of the amountsof power required by the active interface circuits.
 2. Apparatusaccording to claim 1 in which the regulating means is so arranged, inuse, that it monitors the respective voltage applied by each activeinterface circuit to its respective line, controls the Dower supplied tothe interface circuits by controlling the voltage applied to the inputsof the interface circuits.
 3. Apparatus according to claim 1 in whichthe apparatus is adapted to be connected to a source of dc power. 4.Apparatus according to claim 1, in which the power regulation meansincludes a switch mode power supply unit operable to reduce the voltagesupplied by the source to the interface circuits.
 5. Apparatus accordingto claim 1, in which the interface circuits are mounted on a single cardfor mounting in a rack.
 6. Apparatus according to claim 5, in which thecard is one of a plurality of such cards, and the regulating means isone of a plurality of such means each associated with a respective oneor more cards.
 7. A telephone exchange having power supply apparatus inaccordance with claim 1 connected to a plurality of remotely situatedtelephones along corresponding lines, and to a dc power source.
 8. Anexchange according to claim 7 in which the power supply apparatus has aplurality of cards, each having a respective set of interface circuitsand all the lines connected to the circuits on each cards being oflengths which lie in a respective one of a plurality of differentranges.